Recovery of CH4 from coal mine model gas mixture (CH4/N2) by hydrate crystallization in the presence of cyclopentane

This work presents how water containing cyclopentane (CP) can facilitate the recovery of methane from the CH4/N2 coal mine gas mixture through hydrate crystallization. Gas hydrate formed in the CH4/N2/CP/H2O system has more favorable equilibrium conditions than that formed in pure water with the same gas mixture. The incipient equilibrium conditions at 4.0 and 13.0 wt% CP were measured and are reported. The dynamics of hydrate formation using a 30 mol% CH4/N2 gas mixture were studied at 283.4 K, two CP concentrations (4.0 and 13.0 wt%) and at 2.6 and 3.8 MPa. These hydrate formation pressures correspond to 2.3 and 3.5 MPa overpressures (driving forces) respectively. It was found that CH4 is preferentially incorporated into the hydrate crystal compared to N2. However, N2 competes with CH4 to enter the hydrate crystal at the higher driving force. The CH4 recovery substantially increases as the CP concentration is increased from 4.0 to 13.0 wt%. The experimental condition of 283.4 K, 2.6 MPa, and 13.0 wt% CP is preferred for the separation of CH4 from the 30 mol% CH4/N2 gas mixture. The CH4 recovery obtained at this condition is approximately 46.1% and the corresponding CH4 content in the hydrate is 47.2 mol%. A methane-rich stream (72 mol% CH4/N2) is obtained after two stages of gas hydrate formation in the presence of cyclopentane.